Wet Paint Films
The New Jersey Zinc Company
160 Front Street, New York
Wet Paint Films
\. II. I'M M), I'll. I).
i i Fohna 1 lopkiim I Diversity
■r>. The Nevi Jersey Zinc Company
The \r\\ Jersey Zim I ompan}
! Ihhed HUH*
160 I 3 Vew York
The \«u Jersey Zini 3
■ Pittsburgh I level md - I
of Wet Paint Films
By A. H. Pfi m.
Associate Professor c»f Physics, Johns liopkin I
Research ] boi >'ory. The New I * ompany
"T" \ ci.nnr, imii wilh the stud> of the hiding-power of paints, a^
revealed l>> the cryptometer, 1 the nerd of an instrumenl for deter-
mining the thickness oi a singl< coal of we1 painl has made itself
felt. The Geld "I application of such an instrumenl would he wide:
I knowing the total thickness of layei uecessar) for complete hiding, a
kiiMW ledge <>l i he thickness of one coal would make possible a determina-
tion of \\w "number of coats"; (2) exposure tests could h«- carried ou1 on
paint or varnish films of known thickness; (3) the shrinkage of paint
films upon drying could be studied l when the air-brush is used, the
thickness of coating mighl be controlled; 5 the hiding-powers of paints,
such .i- I'.ii whites' .nid <n|i| wain- paints, which experience a marked
change upon drying, mighl I e determined; 6) the skill of a painter in
;ipl l> ii -. iinifnim mu\> eould be put to a severe test.
The usual method of evaluating the thickness of a we1 paint-film
involves the rather laborious procedure of determining, first, the specific
-v it > of the paint. Subsequently . a krmw n area of plate-glass is coated
with paint. The can, brush and painl are weighed t«» an accuracy of
0.0] gr. both before and after painting >o as to > ield the mass <>t" the paint
applied. \ simple calculation yields the average thickness of ihe paint-
film. ^side from the length of time involved in carrying ou1 such a
determination, the principal objection to this method i-- that it cannol be
used if the constitution and hence, the specific gravity) of the [taint i^
unknown. Furthermore, the method cannol be used in checking up the
,n I ii. il pi'i I'm manee <>t* a painl. i
I hese difficulties are o\ ei i i «ne in the instrument about to be described.
Advantage is taken of the circumstance that paint is a plastic solid, 2
w h( se "\ ield-A alue" is greater t ban thr capillary forces introduced through
the use of the instrument. The principle ma) he illustrated most
tpfund: Jour Franklin Insl Nw 1919 p S75.
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simply by considering a uniform layer of paint upon which a glass plate,
with one edge slight 1\ raised, i- placed. As shown diagrammatical I >
in Fig. 1. the volume of paint, originally in the layer ABCDA, is crowded
into the wedge form ABEA. Since the wedge-angle (a) i^ \ er> small, the
thickness it) of the original film is given by the relation:
where L is the length of the wedge wet b\ the paint, i.e.. the distance
AE in Fig. 1.
Since the wedge calls for essentially plane surfaces to which the paint
is applied, and since, furthermore, some of the paint is squeezed out from
underneath the we nimetrical wed.', u.i* chosen. This j. ^implv
a convex, spherical surface which is forced through the paint layer.
\- shown in Fig. 2. the volume of paint originally in the layei A B( A.I is
forced into the meniscus AEFGBA. If the diameter (D) of the circular
paint-spol be measured, and if* the radius of curvature (R) ..1' the convex
surface be known, then. L> performing a simple integration, ii is found
that the thickness / of the original paint-film is given by the relation
The final form giveD the instrument i- shown in Fig. 3 \ convex
lens /. whose lowei surfac* has a radius of curvature ol 25 cm. is mounted
in a shoii tube T. which slides freely in an outei tube T rhe compres
sion -pring- S keep the convex surface out of contact with the paint-film
until pressure is applied to the top ol 7 . This instrument, which is to
be designated a- tie- 'Taint-Film Gauge," is simply rested on a painted
surface, and the lens i- forced down as far .«- ii will go. I pon releasing
the pressure before renun ii \ ir< ular spol is Left on tl
the painted surface. The diametei oi the spol on the ga
red to 0.1 of a millimetre. \\ the spol i- elliptical, the mean of
the major and minor axes is d< termined. \ series oi readings on at least
spots, judiciously distributed ovei a given area, yields a fair value oi
the i .,!,,, |u referring to the table printed below,
be paint-film in mm. and the number of square fe< I
Mil NIK KNESS Ol \\l I r\l\ 1 FILM
which would be covered h> one gallon of painl m,i\ be evaluated ai once.
d = Diameter of paint spot im lens i in nun
^ =Thi< kness of painl turn in mm.).
Thickness in mm.
. .28900 .
\ mimher of experiments were carried ou1 to check the accuracy of the
Paint-Film Gauge. In each case a known mass of paint was spread uni-
formly over a definite area of plate-glass, and the thickness of lilm was
calculated in the usual manner. Different gauges ;tn<l thickness of layer
were used. The results are briefly presented in the following table:
lilm Ihirkin— !>v rah iilah.ni O.Ol It mm.
Film thirknr-s h\ Paint-Film (iaup- 0.0 118
Considering the roughness of the measurement, the agreement is sur-
prisingly good. EqualK salisfarlorih results were obtained <m varnish
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films. The differences between the two sets of measurements rarely
exceeded 3 per cent.
Having established the reliability of this instrument, it was sought next
to determine whether or not different paints had different spreading rates.
\\ bile it is obvious that a gallon of paint containing much turpentine and
other "thinners" will be spread over a much larger area than an equal
volume of a thick, creamy paint, it seemed worth while to test the
performance of various painters with the same kind of paint. Accord-
ingly, three different kinds of painl were prepared and each painter ap-
plied these to a clap-board panel whose surface had been previously
primed so as to be impervious to the layer applied. The panel stood in
an essential!} vertical position, and each paintei was asked to appl} a
characteristic "second coat." The results obtained are shown in the
l li.ir.n l.-r <>i Painl
i 1 1
Lit honour > , -
No. L-jZinco^dde 6L5 per cent.
1 Qseed oil 35 per cent.
No. 2.\ White lead &***<***■
Linseed oil. :\~ per cent.
White lead, 7;.
Linseed oil, 22 jmt cent.
I .1 q.ft.
B foi | '.lint
Painter V Ends a maximum spreading rate for paint No. I.
\<». 2, and < foi paint \o. 3.
The paints were designated as having respectively "no tooth,' 1 mi
dium tooth" and "much tooth." Whether oi not these characteristics
affect the spreadii , tain f rom 1 1 1 . u itg \\ | M |,.
N, ' ,v probably, a theoretical value of spreading-rate might be obtained
thr< \ tb plastonn U i ms clear, that, in i
■'• ' diffen i | between individual painters than t> tween paints.
While hiding-powei and coloi are definite characteristics ol a paint, the
THE THICKNESS Ol W! I l> UNT FILMS
spreading-rate depends \er\ largely upon the painter and is. therefore,
in practice a variable quantity.
The shrinkage of we1 paint-films ma} be determined readily b\ measur-
■^ L —-^ i **^
ing tli*' thickness of the wel film h\ means of the Paint-Film Gauge and
thai of the di > film b> means of a micrometer. While a detailed stud} of
this problem i- of interest, ii was deemed of less immediate importance
than the one dealing with the effed of shrinkage on hiding-power particu-
larly for those paints which become much brighter upon drying. This
problem was attacked by coating a piece of plate-glass (11 x L3 cm
with aflat-white paint upon which black spots, irregularly spaced, wen
marked. The en I ire surface was given a hea\ ) coating of \ arnish, which
was sandpapered smooth when dry. \ metallic strip. o.;s nun. thick.
was glued to this surface a1 the edge of the plate, so thai when a pool of
pain! was poured on the plate, it could be gi\en ,1 wedge-shaped form by
leveling off the surface with a straight-edge (steel rule). The thickness
() f the wedge would obviously \ar\ from zero to 0.3 nun. While the
paint is wet. record is taken of the position of the la^t spol v isihle and the
film-thickness adjacent to this spot is determined wilh the Paint-Film
Gauge. These data yield the hiding-power of the wet paint. Several
other measurements of thickness al various distances from the metal
^trip are madr -. .1- in obtain a relation between film thickness and dis-
tance from metallic strip. When the paint i- dry, the lasl spol \ i^ihb*
is again -mi^lil . and the thickne— • »f the I paint-film, formerly overlying
ihiv spol is calculated. This, of course, yields the hiding-power of the
dry paint. While this mode of ] :edure is less accurate than thai result-
ing from the use of the cryptometer, ii nevertheless makes possible a
determination of hiding-powei to an ao ura< ) of 5-10 per cent, and solves
a type of problem to which the cryptometer < mnotbe applied.
For lack of time, such numerical determinations were not made —
only qualitative results were recorded. Separate strips of glass were
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also coated with the various paints which were tested for shrinkage.
The drying was done rapidly over a radiator. Measurements of bright-
ness on the we1 and dry films were made with the Colorimeter. 4 The
results obtained are shown in the following table:
< lommercial i old-water paint
Zn 0, tui penl ine
Zn < *. gelatine water
Zn < '. \ arnisfa
Zn < ). Linseed nil Dri<
Commercial flat wall-finish
pel < ••nt
< h.m-f in In. I nip-power
\ «t> great increase
Marked in< rease
\t» < Ii.ui^.'
Slight de< case
hi spite of large variations in shrinkage values, the hiding-powei does
no1 \ar\ with shrinkage but with brightness changes. This conclusi.m
applies util\ in case that no chemical changes, producing a darkening of
the paint-film, sel in during the process of (Irving. In the majority of
the above paints, the drying simply involves the evaporating of a fluid.
It air-pockets are left, an increase in brightness results because of the
-mailer refractive index of air. While shrinkage undoubtedly has some
effect, its influence is entirely overshadowed by the changes in brightness.
I \ now instrument, the Paint-Film Gauge, has been devised to meas-
ure the thickness ol we1 paint-films. Th.- attainable accuracy is better
than 3 per cent.
2. The spreading-rates oi differenl types of paints were determined b\
means f the Paint-Film Gauge. Variations introduced h> the painters
iter than those introduced l.\ the paints, hi actual practice,
astanl foi a given type of paint.
Hiding-powers of paints, in particulai those which become markedly
brightei upon drying increase as brightness increases and are but
little affected b> shrinkage of the paint-film.
m the Jouri . i • Id i . \,, fl j [921
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